Little Dynamo

The chickadee is one tough fluff when it comes to weathering winter

02-01-1991
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George H. Harrison

Chickadees are not the suburban wimps that some people think they are," says wildlife ecologist Margaret Clark Brittingham. For three winters in Wisconsin, she kept track of 576 black-capped chickadees. As the small birds struggled against starvation and stinging cold, they earned her respect: "They are tough survivors that live close to the edge of life."

While roughly half of the continent's bird species head south for the winter, black-capped chickadees stay in the northern tier of the United States and southern Canada, usually in the same 20 acres or so they occupied in summer. Perhaps the most studied of winter songbirds, chickadees are giving us insights into how little balls of fluff with bare feet withstand severe weather.

To cope with cold, a chickadee needs at least 20 times more food in winter than it does in summer. Brittingham calculates that in mild winter weather, a chickadee must eat the equivalent of 150 sunflower seeds a day to stay alive. When the temperature drops to zero degrees F, the bird must find more than 250 sunflower seeds daily, the equivalent of 60 percent of its body weight.

Thus chickadees' lives depend on their efficiency as foragers. They roam the winter woods in groups of five to nine, hunting for insect eggs and seeds. Chickadees investigate every nook, prying under bark, into cracks and under the eaves of houses. They seem to hang upside down as much as they perch right-side up.

A chickadee band often joins a larger hunting party that includes tufted titmice, white-breasted nuthatches, brown creepers and downy woodpeckers. This mixed flock, an uncommon phenomenon among bird species, can scour the snowy landscape for food more efficiently than chickadees do alone. Birds that eat the same diet learn about food sources from other members of the flock and find safety in numbers.

When a mixed flock descends on a backyard feeder, the human hosts may decide the feeding station is the mainstay of this profusion of hungry birds. Some dedicated backyard birders forego winter travel for fear that birds will starve without the accustomed handouts.

These folks may take comfort from Brittingham's study of chickadees at the University of Wisconsin-Madison. She fitted chickadees with color-coded bands so she could identify individuals. From 1982 to 1985, she compared the survival rates of 418 birds living near feeders and 158 that foraged entirely in the wild. The chickadees that visited feeders, according to Brittingham, obtained only 20 to 25 percent of their daily energy requirement there, picking up the rest from woods and fields. During normal winter weather, chickadees did not become dependent on feeder food. When people withheld seed from the birds, the chickadees survived at the same rate as those that never visited feeders.

Temperatures below 10 degrees F, however, changed the story. In vicious cold, according to Brittingham's study, backyard sunflower seeds nearly doubled the survival rate of her subjects compared to birds that forage in the wild. When she weighed the banded birds, Brittingham found that feeder-fed chickadees were a little fatter than those eating only food from the wild.

Although chickadees may need supplemental food only in very cold weather, they prove frequent visitors at feeding stations. At the Schlitz Audubon Center in Milwaukee, naturalist David Stokes places straw-filled dummies on a bench next to the center's bird feeders and piles sunflower seeds on the dummies' broad-brimmed hats. When children visit the center, they take turns wearing a feeder-hat and sitting on the bench in place of a dummy. Holding as still as possible, the children soon have the thrill of a one-third-ounce chickadee making a surprisingly hard landing on their heads.

Nature-watchers have long known that feeding chickadees in winter brings out the birds' legendary tameness. Birder John Woodcock, for example, published an account in 1913 of a chickadee so tame it would often swing head-downward from the bill of his cap. "It was rather amusing when I took the .22 rifle to shoot rabbits," he reported. "After the first shot was fired, I was attended by several chickadees. They made aiming almost impossible, for every time I raised the rifle, one or two birds would perch on the barrel, completely hiding the sights."

Whether they pluck seeds from a doting human's hand or a wild weed, chickadees build up reserves of fat to draw on during the night, according to a study by Cornell graduate student Susan Chaplin. Brittingham noticed this phenomenon also. "The thing that really impresses me about chickadees," she says, "is their metabolism. We weighed birds early in the morning and found that they had virtually no body fat. Yet, the same birds examined in the afternoon of the same day were bulging with fat."

Ornithologists now suspect that chickadees conserve these precious fat reserves at night by allowing their body temperatures to drop nearly 20 degrees below their daytime temperature (around 108 degrees F). With this dip in metabolism, called hypothermia, the resting chickadee can wake up the next morning with a modest surplus of energy-rich fat.

Chickadees fight the cold with more than metabolic tricks. Like other birds, they tense tiny muscles in their skin that raise feathers and trap an insulating layer of air, much like a down quilt. They develop a generous layer of fluff; their winter plumage has 25 to 30 percent more feathers than their summer garb.

In addition, chickadees can shiver. The motion quickly turns energy into heat, but the energy must soon be replaced through food or the birds will die.

Then there is the matter of exposed feet and bills. A bird's bill is not skin but a substance like horn that doesn't freeze easily. Its feet, however, are sensitive, and, on extremely cold days, a bird may pull one foot at a time up into its breast feathers or sit down on both feet.

"When the temperature dropped below minus 20 degrees F," Brittingham says, "we noticed that chickadees stopped searching for food because the energy expended to find food at that temperature is greater than the energy they obtain from the food they find. When that happened, they simply slowed down, fluffed up and waited for warmer weather."

Despite all their adaptations, many chickadees do not survive winter. More than 70 percent of all young chickadees are not alive one year after they hatch. Most die during their first month of life, but others are victims of predation, starvation and exposure to the cold. The fact that any live to greet the spring is an amazing story of survival.

Field Editor George Harrison is the co-author with this wife, Kit, of The Birds of Winter (Random House, 1990).